Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual a média de Abertos que um Fabricante de vekoma tem?
 ###Tabelas: CREATE TABLE women's_doubles:_10_finals_3_titles_7_runner-ups (women's_doubles:_10_finals_3_titles_7_runner-ups_id AUTO_INCREMENT PRIMARY KEY, Outcome VARCHAR, Year INT, Championship VARCHAR, Surface VARCHAR, Partner VARCHAR, Opponents in the final VARCHAR, Score in the final VARCHAR) CREATE TABLE dates (dates_id AUTO_INCREMENT PRIMARY KEY, Date VARCHAR, City VARCHAR, Country VARCHAR, Venue VARCHAR, Time VARCHAR) CREATE TABLE tallest_proposed (tallest_proposed_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Name VARCHAR, Location VARCHAR, Height feet / metres VARCHAR, Floors VARCHAR, Year Proposed VARCHAR) CREATE TABLE tallest_completed_buildings_and_structures (tallest_completed_buildings_and_structures_id AUTO_INCREMENT PRIMARY KEY, Rank VARCHAR, Name VARCHAR, Height (m) INT, Height (ft) INT, Floors VARCHAR) CREATE TABLE roller_coasters (roller_coasters_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Style VARCHAR, Opened INT, Manufacturer VARCHAR, Themed Area VARCHAR)  
SELECT AVG Opened FROM roller_coasters WHERE Manufacturer = vekoma